It is publicly known that a heteropolyacid catalyst containing molybdenum and phosphorus is effective as a catalyst for use in synthesizing methacrylic acid through gas-phase catalytic oxidation of methacrolein. For example, a method for producing a catalyst for use in synthesizing methacrylic acid is proposed in Patent Document 1 wherein water content of a concentrated or dried material of a slurry obtained by mixing compounds containing catalyst-component elements is adjusted to 5 to 20% by mass followed by heat treatment at 100 to 250° C. A method for producing a catalyst for use in synthesizing methacrylic acid is proposed in Patent Document 2 wherein water content of a concentrated or dried material of a slurry obtained by mixing phosphomolybdic acid and compounds containing other catalyst-component elements is adjusted to 5 to 20% by mass followed by heat treatment at 100 to 300° C.
Patent Document 1: Japanese Patent Application Laid-Open No. Sho58-51,943
Patent Document 2: Japanese Patent Application Laid-Open No. Sho58-112,050